The epidemic of post-vaccinal hepatitis in the U.S. Army in 1942 created an unusual opportunity to examine the relationship between hepatitis B virus (HBV) and primary hepatocellular carcinoma (HCC). Federal record systems made it possible to carry out a serologic study (New Engl. J. Med. 316:966, 1987) that showed the outbreak to have been caused by Hepatitis B. These same systems also provided the basis for a 40-year mortality follow-up study aimed at differential mortality with respect to liver cancer and other diseases of the liver, especially cirrhosis.